Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Referring to FIG. 1, there is illustrated schematically a conventional LCOS device 1 including a liquid crystal material 3 sandwiched between a transparent glass layer 5 having a transparent electrode, and a mirror 7 mounted on a silicon substrate 9. The mirror is divided into a two-dimensional array of individually addressable pixels. Each pixel is individually drivable by a voltage signal to provide a local phase change to at least one polarization component of an optical signal, thereby providing a two-dimensional array of phase, optical retardation or polarization manipulating regions. Pre-alignment of the liquid crystal materials within material 3 is provided by alignment layers 11 and 13. These layers generally include a plurality of small grooves induced by rubbing of the polyamide layers or other commonly employed techniques, which align the long axis of the individual liquid crystals to define the slow axis of the liquid crystal material.
LCOS devices are known to suffer diffractive optical losses due to the pixilated electrode structure. Previous efforts to remove the diffractive optical losses associated with the pixel structure involve overcoating the LCOS with a multilayer dielectric coating. These techniques have not been overly successful due primarily to the inherent voltage drop over the large number of overcoat layers required to suppress the pixel structure. This leads to increased loss due to fringing fields (the effective phase pattern produced by the combination of pixels) at the transitions between high and low voltage states. Also the stress of the coating makes it difficult to achieve an optically flat LCOS, which is important for many applications.
To address a separate problem of polarization dependence in liquid crystal devices, US Patent Application Publication 2013/0070326 entitled “Polarization-Independent LCOS Device” to Frisken et al. and assigned to Finisar Corporation discloses use of subwavelength structures to provide a polarization independent LCOS device. This is achieved through manipulation of the phase of the s and p polarization states using a metallic subwavelength structure. While these types of devices are advantageous for maintaining polarization independence, use of a pixilated subwavelength structure necessarily adds to diffraction losses. Further, the addition of a metal layer adds to absorption losses.